Residual-current circuit breakers are designed to protect people and appliances from dangerous residual currents and should be tripped according to the respective, international regulations in the event of residual current occurrences, which is between half the rated residual current and the rated residual current.
Residual-current circuit breakers (RCCB) should, in addition, have a testing facility for testing functionality in accordance to the respective, international, national and/or regional regulations. The stated testing facility usually includes a test resistor and a test button, whereby the operation of the test button will close off the test circuit, and thereby a simulated residual current is produced from one conductor to another conductor past the core-balance current transformer. If the RCCB functions correctly, it will be triggered and the short circuiting of the RCCB severs the conductor of the network being protected.
According to regulations, during the operation of the testing facility, the force provoked in the core-balance current transformer must not exceed the rated residual current by more than 2.5 times the force produced by the rated residual current. In an RCCB with a residual current of 30 mA therefore, according to regulations, a rated voltage factor of up to 75 mA is allowed. A RCCB tripped during the operation of the testing phase therefore will not necessarily trigger safely in the case of a significantly smaller residual current.
The test circuit should, in addition, reliably function in an additional supply voltage area. The test circuit must function therefore at a supply voltage of 85% of the rated voltage factor, as well as in the usual fluctuations of the additional supply voltage and where applicable the line voltage of 10%. Furthermore there are four-pole RCCBs which can also be used as two-pole RCCBs. As a result of the test circuit being of the dimensions stated and this simulated residual current including the rated voltage of the rated residual current of 85%, the simulated residual current is increased by 225%, at the maximum permissible supply voltage. An RCCB which, due to a successful test with a high simulated residual current is deemed functionally safe, still may pose a risk for people or appliances, because it has not been proven that the function of the RCCB will short circuit in the case of a rated residual current.